Recently, concomitant with miniaturization and lightweight trends in electronic appliances, research into energy sources having high density and high energy has been performed intensively. A lithium secondary battery has been proposed as one energy source in the aspect that the higher integration of energy is possible because the molecular weight of lithium used in a lithium secondary battery is very low, but its density is relatively high.
In the earlier developed lithium secondary battery, an anode was fabricated with metallic lithium or lithium alloy. However, a cycle characteristic of such secondary battery using metallic lithium or lithium alloy is reduced significantly due to dendrites generated on an anode in the course of repeated charging and discharging of the battery.
A lithium ion battery was presented in order to solve the problem of dendrite generation. The lithium ion battery developed by SONY Company in Japan and widely used all over the world comprises an anode active material, a cathode active material, an organic electrolyte and a separator film.
The separator film functions to prevent an internal short-circuiting of the lithium ion battery caused by contacting of a cathode and an anode, and to permeate ions. Separator films generally used at the present time are polyethylene (hereinafter referred to as “PE”) or polypropylene (hereinafter referred to as “PP”) separator films. However, the lithium ion battery using the PE or PP separator film has problems such as instability of a battery, intricacy of its fabrication process, restriction on battery shape and limitation of high capacity. There have been attempts to solve the above-mentioned problems, but there is no clear result until now.
On the contrary, a lithium polymer battery uses a polymer electrolyte having two functions, as a separator film and as an electrolyte at the same time, and it is now being viewed with keen interest as a battery being able to solve all of the problems. The lithium polymer battery has an advantage in view of productivity because the electrodes and a polymer electrolyte can be laminated in a flat-plate shape and its fabrication process is similar to a fabrication process of a polymer film.
A conventional polymer electrolyte is mainly prepared with polyethylene oxide (hereinafter referred to as “PEO”), but its ionic conductivity is merely 10−8 S/cm at room temperature, and accordingly it can not be used commonly.
Recently, a gel or hybrid type polymer electrolyte having an ionic conductivity above 10−3 S/cm at room temperature has been developed.
K. M. Abraham et al. and D. L. Chua et al. disclose a polymer electrolyte of a gel type polyacrylonitrile (hereinafter referred to as “PAN”) group in U.S. Pat. No. 5,219,679 and in U.S. Pat. No. 5,240,790 respectively. The gel type PAN group polymer electrolyte is prepared by injecting a solvent compound (hereinafter referred to as an “organic electrolyte”) prepared with a lithium salt and organic solvents, such as ethylene carbonate and propylene carbonate, etc. into a polymer matrix. It has the advantages that the contact resistance is small in charging/discharging of a battery and desorption of the active materials rarely takes place because the adhesive force of the polymer electrolyte is good, and accordingly adhesion between a composite electrode and a metal substrate is well developed. However, such a polymer electrolyte has a problem in that its mechanical stability, namely its strength, is low because the electrolyte is a little bit soft. Especially, such deficiency in strength may cause many problems in the fabrication of an electrode and battery.
A. S. Gozdz et al. discloses a polymer electrolyte of hybrid type polyvinylidenedifluoride (hereinafter referred to as “PVdF”) group in U.S. Pat. No. 5,460,904, The polymer electrolyte of the hybrid type PVdF group is prepared by fabricating a polymer matrix having a porosity not greater than submicron, and then injecting an organic electrolyte into the small pores in the polymer matrix. It has the advantages that its compatibility with the organic electrolyte is good, the organic electrolyte injected into the small pores is not leaked so as to be safe in use and the polymer matrix can be prepared in the atmosphere because the organic electrolyte is injected afterwards. However, it has the disadvantages that the fabrication process is intricate because when the polymer electrolyte is prepared, an extraction process of a plasticizer and an impregnation process of the organic electrolyte are required. In addition, it has a critical disadvantage in that a process forming a thin layer by heating and an extraction process are required in fabrication of electrodes and batteries because the mechanical strength of the PVdF group electrolyte is good but its adhesive force is poor.
Recently, a polymer electrolyte of a polymethylmethacrylate (hereinafter referred to as “PMMA”) group was presented in Solid State Ionics, 66, 97, 105 (1993) by O. Bohnke and G. Frand, et al. The PMMA polymer electrolyte has the advantages that it has an ionic conductivity of 10−3 S/cm at room temperature and its adhesive force and compatibility with an organic electrolyte are good. However, its mechanical strength is very poor, and accordingly it is unfeasible for the lithium polymer battery.
In addition, a polymer electrolyte of a polyvinylchloride (hereinafter referred to as “PVC”) group, which has good mechanical strength and has an ionic conductivity of 10−3 S/cm at room temperature, was presented in J. Electrochem. Soc., 140, L96 (1993) by M. Alamgir and K. M. Abraham. However, it has problems in that a low-temperature characteristic is poor and a contact resistance is high.
Recently, in order to complement disadvantages of lithium ion batteries and lithium polymer batteries, new methods have been attempted. Among them, U.S. Pat. No. 5,681,357, U.S. Pat. No. 5,688,293 and U.S. Pat. No. 5,834,135 to M. Oliver et al. disclose a fabrication method of a secondary battery. Such method comprises steps of casting a solution in which a polymer such as PVdF, etc. is dissolved in an organic solvent or an organic electrolyte onto a PP or PE separator film used for lithium ion batteries to obtain a separator film system, locating the obtained separator film system between an anode and a cathode, making the resultant into one body by a heat lamination process and then injecting an organic electrolyte solution. However, in the method, because a polymer solution is cast onto a PP or PE separator film, deformation of the PP or PE separator film is caused and pores of the separator film is closed. In addition, contact is insufficient because the separator film and electrodes are made into one body by a heat lamination process, and therefore, a contact resistance may be increased. Due to those disadvantages, charge and discharge characteristics of lithium secondary batteries are poor and life characteristics of batteries may be lowered.
U.S. Pat. No. 5,691,005 and U.S. Pat. No. 5,597,659 to Kenichi Morigaki et al. disclose a method for improving a cycle life of a batteries by restraining the generation of dendrite of lithium when metallic lithium or a lithium alloy is used as an anode. Such effect is achieved by injecting a UV (hereinafter referred to as “UV”) curable oligomer or monomer into a PE separator film and then irradiating UV rays onto the resultant, to generate a gel-polymer electrolyte at pores in the PE separator film. However, in such method, although it is possible to improve cycle life, a resistance is increased compared with the one in which an organic electrolyte solution is impregnated into the PE separator film because a polymer electrolyte is injected into the pores of the PE separator film. Accordingly, charge and discharge characteristics of the lithium secondary batteries are lowered. In addition, its adhesive force with an electrode is inferior, and therefore, a fabrication of a battery becomes intricate.